Telephone system



Aug. 23, 19%. R. TAYLOR ETAL ZAQSfiZQ TELEPHONE SYSTEM Filed Aug. 19, 1943 '2 Sheets-Shet i INVENTORS Y Y 5 v REGINALD TAYLOR eeone; THOMAS BAKER ATTORNEY Au R. TAYLOR ETAL. ,5,

TELEPHONE SYSTEM -Fild Aug. 19, 1945 E 2 Sheets-Sheet 2 I I t 3 lNVENTORS REGINALD TAYLOR GEORGE TH OM$ KER Patented Aug. 13, 1946 TELEPHONE SYSTEM Reginald Taylor and George Thomas Baker,

Liverpool, England, assignors, by mesne assignments, to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application August 19, 1943, Serial No. 499,194 In Great Britain September 17, 1942 18 Claims.

The present invention relates to telephone or like systems and is more particularly concerned with systems in which it is desired to effect the setting of automatic switches or the like by trains of impulses transmitted over considerable distances.

In arrangements of the type normally employed in automatic telephone systems in which control is efiected by repeatedly interrupting the flow of direct current over the control circuit, difficulties are experienced as the length of line increases owing to the distorting effect produced by the inductance and capacity of the line. Another difiiculty which arises under long line dialling conditions is due to the impulse distortion produced by speech transmission condensers in association with impedances which are connected to the speech conductors. The removal of these reactive elements from the line prior to and during dialling out has hitherto involved considerable circuit complexity as well as impulse storage and regeneration. The chief object of the invention is to provide improved arrangements enabling direct current impulsing to be used satisfactorily over very long lines by taking suitable steps to minimise the distortion which would otherwise be produced.

According to one feature of the invention, in a telephone or like system having connections set up over a junction in response to impulses trans-- mitted from an operators position, at the incoming end of the junction the circuits are normally conditioned for the reception of impulses and speaking conditions are set up in response to an operation performed by the operator on the completion of impulse sending.

According to another feature of the invention, in a telephone or like system having connections set up over a junction in response to numerical impulses transmitted from an operators position, each impulse is transmitted by the momentary completion of a direct current circuit over the junction while the establishment of current flow over the junction in the opposite direction in response to an operation performed by the operator on the completion of impulse sending serves to introduce speaking conditions in the incoming equipment.

A further feature of the invention is that in a telephone or like system having signals transmitted over a junction by the intermittent completion of a direct current circuit thereover, the equipment at the incoming end is initially prepared for operation by the momentary completion of a direct current circuit over the junction which serves to efiect the operation of a gridcontrolled gas discharge tube which is adapted to be intermittently operated subsequently in response to the transmission of numerical impulses.

According to another feature of the invention, in a telephone or like system having signals transmitted over a junction by the intermittent completion of a direct current circuit thereover, the equipment at the incoming end includes a grid-controlled gas discharge tube which operates in response to each numerical impulse to control the repetition of that impulse to an automatic switch and also operates in response to a signal indicating that impulse sending is completed to introduce speaking conditions in the incoming equipment.

The invention will be better understood from the following description of one method of carrying it into effect which should be taken in conjunction with the accompanying drawings comprising Figs. 1 and 2, which should be arranged with Fig. 1 on the left. Fig. 1 shows the equipment associated with an operator's sleeve control position at the outgoing end of a long junction line, while Fig. 2 shows the corresponding receiving equipment at the incoming end.

The general arrangements are similar in some respects to those described in our application Serial No. 446,900, filed June 13, 1942, where another solution of the general problem is disclosed, this however involving th use of mechanical regenerator at the. receiving end which is not necessary with the arrangements according to the invention. The setting up of a normal call will first be traced through briefly.

When the line in question is free, relay SLR, Fig. 1, will not be operated and will co-operate with similar relays associated with other lines to complete a chain circuit for giving a visual indication of the next line to be used. If this is the line shown, when the operator inserts her plug to seize the line, no circuits are completed over the line conductors but battery applied to the sleeve of the jack J operates relay SR which in turn at its contacts srl energises its relief relay SS. Thereupon at contacts ssl a circuit is completed for relay SLR which alters the visual idle indicator circuits so as to mark the next line as free, While at contacts ss2 flicker earth over lead It! is connected up over the low resistance upper winding of relay SE to flash the operators supervisory lamp, thereby indicating that the equipment is ready for further operation. Relay SR remains operated under this condition and in condition owing to high resistance of the resists ance battery feed to line.

at contacts rp and 25 completes a battery feeding circuit over the junction including resistances YK, YL, YH and YJ, contacts mrB and mrl, windings of relay RS, inductance Tl which with condenser QD and resistance YG form a lowpass filter, and the primary winding of transformer T2, Fig. 2, at the distant exchange. Relay RS operates and at contacts rsl completes a circuit for relay DT to the earthed contacts ss3, while at contacts 1'32 it holds relay RP independently of relay RR. Relay DT in operating, at contacts dtl and (1253 completes locking circuits for itself and for relay RP and at contacts c2152 disconnects the flicker earth from the jack sleeve and prepares a circuit for relay MR. This relay cannot operate owing tothe high resistance introduced in the sleeve circuit at the operators position, but the resistance of relay MR is sum- Relay RR in operat ing, at contacts rrl brings up relay RP which cient to extinguish the operator's supervisory lamp. Relay DT also at contacts di l shunts out rela RR and at contacts dt5 connects battery via resistance YE to the other winding of relay A which now operates. Relays A and .DT together at contacts a1 and CH6 shunt. the battery...

connection to the junction so that current falls to zero in the winding of the distant transformer The tubes GTA and GTB in the incoming equipa grid controls the striking of the tube but thereafter exercises no further control, the tube being extinguished only by a change in the anode circuit. These tubes are preferably of the hot cathode argon-filled type. It will be noted that a circuit is normally completed for the filaments of the tubes in series in a circuit including resistances YN, Y0 and YR. The grids of the tubes are connected to separate windings of transformerT2 in such manner that one tube is caused to strike by a fall in the line current and the other tube by a rise in the line current. The purpose of the arrangement shown whereby the negative bias of the tube GTA is derived by way of resistance YN which equals the hot resistance of the filament of either tube GTA or GTB is to ensure that the standing negative grid bias on each tube with respect to its filament and therefore the striking voltage will be substantially the same in spite of the fact that the filaments being in series are at slightly different potentials. The purpose of the condensers QF and QG and associated resistances YM and Y? will be dealt with later.

The initial closing of the line circuit on the operation of relay RP at the outgoing end produces a momentary voltage in the secondary windings of transformer T2, the effect of which isto reduce the negative bias on the grid of the tube GTB but it does not strike at this time owing to the fact that its anode circuit is not yet closed. Shortly thereafter when relays A and DT operate, the current in the line circuit is again reduced to zero and a pulse is then produced in the opposite direction by the transformer T2. This has the eifect of reducing the negative bias ment are gas discharge tubes of the type in which.

tube into operation, whereupon relay PD is energised. This relay then at contacts poll connects the discharged condenser QH in series with its winding whereupon the tube is at once extinguished by the substitution of negative for positive potential and as the condenser becomes charged the current falls until relay PD can no longer remain operated. With this arrangement the period of operation of relay PD may be readily adjusted by suitable choice of the value of condenser QI-I. Relay PD also at contacts 11:12 completes a circuit for relay KO which looks up over its contacts kol, completes a, loop forward to the incoming first selector at its contacts 7e02, prepares circuits at contacts 7e04, R05 and bot for relays PS, PB and KN and at contacts 7:03 completes a circuit for relay CD. Relay CD is slow-to-operate and consequently contacts cd2 do not complete a circuit for relay KN at this time since relay PD will'have released before relay CD, operates. No further changes occur until the operator commences to dial.

When the dial is moved off-normal, the battery connection to the T and R lines at the operators position is removed and these line conductors are left directly connected together over the dial springs. When these springs first open, relay A releases and at contacts 0.! removes the shunt across the battery connected to the junction line circuit so that line current again flows. This has the effect of reducing the negative potential on the grid of tube GTB and since its anode circuit is now completed by the operation of relay KO, it strikes and operates relay PS. Dhis in turn at contacts psi brings up relay PB which at contacts pbl connects relay P2 to the anode of tube GTA in place of relay PD, at contacts pbZ locks up, at contacts 173 releases relay PS and extinguishes tube GTB, at contacts p124 repeats the impulse by opening the circuit to the first selector, and at contacts pbS transfers relay CD to the P conductor extending back from this switch so that it is held up by the guard earth which was connected thereto when the loop was extended forward.

When the impulse springs again close, relay A reoperates and battery is again shunted off the line. Tube GTA is now triggered into action with the result that relay PZ operates and at contacts p22 effects the release of relay PB. Relay PB again closes the loop to the first selector at contacts 12b4, transfers relay CD to its original circuit at contacts M25, and at contacts pbl and p173 again prepares circuits for relays PD and PS, tube GTA being extinguished and relay PZ releasing on the opening of the contacts pbl. In response to the next break, relay PS is again operated and brings up relay PB and when the loop is again closed relay PZ operates and brings down PB. This operation continues during the difierent trains of impulses which are accordingly repeated to the first and subsequent selectors and the position at the end of dialling is that only relays K0 and CD remain operated in the incoming equipment, relay KO being locked over contacts sk and 7001 and relay CD being held locally over contacts 7e03, p126 and 7on3.

The operator now restores th dialling key, whereupon the high resistance introduced into the sleeve circuit at her position is short-circuited and this causes the operation of relay MR. This relay thereupon locks up over contacts mrZ to battery via resistance YB, prepares answering supervisory circuits at contacts mr3, mr l and mr5,

opens the circuit of relay DT at contacts mrl, reverses the battery connections to the junction at contacts mrfi and mi? and opens the shunt at contacts mr8 so that current flow over the junction now takes place but in the reverse direction to that previously flowing.

Consequently, at the distant end, the impulse generated by transformer T2 at the distant exchange is in the same direction as that previously produced by cessation of current flow over the line, that is to say, in the sense to cause the operation of tube GTA. As a result relay PD is again operated and since relay CD is energised at this time, a circuit is now completed for relay KN from earth over contacts cd2 and lcofi. Rela KN locks up at contacts .cnl dependent upon relay KO, opens the circuit of relay CD at contacts 7:713 and at contacts 7on2 completes a shunt across the speaking leads by way of resistance YS to prevent the release of the established connection during the changeover operation. On the release f relay CD after its slow period, relay SK is operated over contacts ccZ'l and 1on3 and thereupon locks up over contacts 0031, p126 and $703 to the P conductor. At contacts ski and slc2 it disconnects the primary winding of the transformer T2 and switches the line through, while at contacts sled it opens the circuit of relay KO. This relay releases after its slow period and in turn releases relay KN. The speaking leads are then switched straight through at the incoming relay set and the only relay remaining operated is relay SK which is locked up to the P conductor.

At the outgoing end, relay DT, in releasing in response to the operation of relay MR, at contacts diZ further prepares a circuit for giving called party supervision to the operator. At contacts (its the short-circuit is removed from relay RR, which however cannot re-operate since the battery connection to the T and R lines was broken when the dial key was restored. At contacts dt3 one looking circuit for relay RP is opened and the continued holding of this relay is now dependent on relay RS. When relay SK at the incoming end operates and initiates the release of relay KO, during the slow release of relay KO there will be no loop holding circuit for relay RS at the outgoing end, and this relay will thereupon release and in turn bring down relay RP. At contacts we and M05 relays D and I are now connected across the outgoing loop and on release of relay KO at the incoming end relay I will operate in series with the battery feeding relay of the final selector at the distant exchange. Relay I in operating at contacts 2'! energise the polarising winding of relay D, at contacts 12 extends full earth back via the upper low resistance winding of relay SR to the sleeve circuit so as to light the operators supervisory lamp and indicate that connection has been made with the called party but that he has not yet answered the call, while at contacts 13 the battery connection via resistance YF and the right hand winding of relay A is removed from the R conductor, this arrangement being used as will be later described for through supervision purposes cases where the originating call has arrived over an incoming junction.

When the called party replies, battery reversal takes place over the line from the distant final selector, whereupon relay D operates and at contacts dl extinguishes the operators supervisory lamp and at contacts d2 restores the connection of resistance battery potential to the R conductor for through supervision purposes. The operator now releases her cord circuit speaking key so as to cause a through circuit to be provided in the cord circuit between the calling and called parties, and conversation now proceeds.

At the end of conversation if the called party hangs up first, relay D will release and at contacts (ll will re-light the operators supervisory lamp, while at contacts d2 the resistance battery potential will be removed from the R conductor so as to give through supervision to a distant originating operator in cases where the incoming call has arrived over a junction. The sleeve control operator may now wait for clearing supervision from the originating exchange and when she withdraws her plug from the jack J the sleeve relay SR will be released and will bring about the release of relays SS and MR. Relay MR in releasing at contacts mr5 opens the loop via relays D and I to the outgoing junction thereby initiating the release of the automatic switch train in the distant exchange. When this has been effected earth is removed from th P conductor to allow relay SK to release, whereupon both the outgoing and incoming equipment is ready for further use.

In case the Wanted subscriber is found to be busy, busy tone and ilash is returned over the speaking leads from the final selector and relay I flashes in response to the busy flash signals, and at contacts i2 flashes the sleeve control position operators supervisory lamp, while at contacts battery via resistance YF is intermittentlyv applied to the R conductor so as to bring about the flashing of the calling supervisory lamp of the originating A operator in cases where the call has arrived over a junction. The calling party is then advised and the connection is taken down, whereupon release of the utgoing and incoming equipments shown is effected in the manner already described.

The case must also be considered in which the desired call is not set up as just described, but is prematurely released by the sleeve control operatc-r withdrawing the plug before completing dialling. In this case relays SR and SS are released by the disconnection of the sleeve circuit, while relay DT which is held from the earthed contacts 883 commences to release slowly. Relay A also releases but relay RP remains held during the release time of relay DT over contacts dtt and mi. On the release of rela A, current is again allowed to ilcw out through relay RS and over the outgoing loop to the transformer T2 so that relay re-operates and maintains the circuit of relay RP independently of relay DT. The current flow over the loop produces an impuls in transformer T2 in such direction as tocause tube GTE to strike at the beginning of a normal dialled impulse, whereupon rela PS operates and in turn brings up relay PB. Relay; PB looks over its contact pbf and at its contacts gobs opens the loop to the selector train. Since this condition persists, the automatic switch train is released in due course earth is removed from the P conductor. Relay CD has been previously held up over this circuit via contacts @735 and E03 and it will now commence to release slowly and when it falls away a circuit is completed over contacts and cell for relay SK. Relay SK in operating at contacts sk opens the holding circuit of relay KC and when this relay releases after its slow period it brings down relay PB at contacts 7e05, whereupon the circuit for relay SK is opened at contacts and it also releases.

Meanwhile at the outgoing end the operation of relay It? with relay SS normal has the effect that at contacts m2 the busying battery via resistance YA maintains relay SLR and at contacts m3 bat.- tery via resistance Y and contacts ss5 is applied to the jack sleeve for engaged test purposes in case this should be required. When relay SK at the incoming end operates, contacts skl and ski open the holding circuit for rela RS at the outgoing end, and since relay RP is dependent upon relay RS, this relay also releases and the outgoing equipment is completely restored to normal. At the incoming end when relay SK releases after having released relays K0 and PB in turn, this equipment also is ready for further use.

It will be understood that if for any reason the automatic switches do not release so that earth is maintained on the P conductor, relay CD will remain held, relay SK will not be operated, and current will continue to flow over the loop to maintain relay RS and in turn relay RP at the outgoing end. Relay RP then maintains relay SLR operated so as to prevent the line being incheated as idle, while the engaged test battery is maintained on the jack sleeve. Moreover, at contacts 7113 the circuit for relay SR is held open so that if the operator should re-insert the plug after only a momentary withdrawal, this relay cannot re-operate until the equipment at the distant end has released and permitted rela RP to fall away.

With regard to the paired condensers and resistances QF, YM and QG, YP which are associated respectively with the tubes GTA and GTB, these serve to integrate the effect of impulses from the windings of transformer T2 so that satisfactory operation from non-steep-fronted received signal waves can be obtained without involving false operation due to parasitic line volt ages of equivalent instantaneous value. These arrangements are dealt with more fully in the specification previously referred to.

It will be appreciated that the impulsing arrangement shown whereby battery is shunted off the line is preferable to the more useful method of opening the line as it permits the rise and fall of current to follow substantially similar curves and thus enables the two gas discharge tubes to operate under substantially identical conditions.

It will be seen also that with the arrangement shown the ratio of the impulses supplied to the automatic switches is dependent on the ratio of the dialled impulses but that this ratio is not appreciably distorted since the factors which produce distortion with relay repetition are not operative on the improved circuit.

What we claim as new and desire to secure by Letters Patent is:

1. In a telephone system, two exchanges interconnected by a trunk line, an operators position in one of said exchanges, means operated under the control of said position to transmit call directing impulses over said trunk line to the other of said exchanges, circuits in said other exchange normally conditioned to receive said impulses and to control the extension of a connection in accordance therewith, said connection including a normally disabled speech circuit, and means for enabling said speech circuit responsive to an operation performed at said position after the transmission of said call directing impulses.

2. In a telephone system as claimed in claim 1, wherein said connection is maintained at times after said trunk line has been released at said po. sition, a relay in said one exchange operated over '8 said trunk line at such times thereby to characterize said trunk line as busy.

3. In a telephone system wherein call-directing impulses are transmitted from an operators position in one exchange over a two-conductor trunk line to another exchange, and wherein said other exchange has numerical switching equipment, circuits in said other exchange normally conditioned to receive said impulses from said one exchange and to control said equipment in accordance therewith, and means in said other exchange controlled by an operation performed at said operators position after said impulses have been sent for preparing circuits in said other exchange for the transmission of speech.

4. In a telephone system, two exchanges interconnected by a trunk line, an operators position in one of said exchanges, means under the control of said position operated to transmit direct current impulses of a certain polarity over said trunk lineto the other of said exchanges and then operated to transmit a direct current impulse of the opposite polarity over said trunk line to said other exchange, means in said other exchange for receiving the impulses of said certain polarity and directing the extension of a connection in accordance therewith, said connection including a normally disabled speech circuit, and means in said other exchange controlled by the impulse of said opposite polarity for enabling said speech circuit.

5. In a telephone system, two exchanges interconnected by a trunk line, an operators position in one of said exchanges, means under the control of said position operated to transmit numerical impulses over said trunk line to the other of said exchanges, a pair of grid controlled gas discharge tubes in said other exchange associated with said trunk line and fired alternately by said impulses, means controlled by said tubes for repeating said impulses thereby tocontrol the extension of a connection in accordance therewith, said connection including a normally disabled speech circuit, means responsive to an operation performed at said position after the transmission of said impulses for again firing the tube last fired by said impulses, and means controlled by said tube for thereupon enabling said speech circuit.

6. In a telephone system, two exchanges interconnected by a trunk line, means in one of said exchanges operated at will to complete a direct current circuit over said trunk line, an interrupter in said circuit at said one exchange then operated to transmit impulses over said trunk line, a grid controlled gas discharge tube associated with said trunk line in the other of said exchanges, said tube operated initially by the completion of said circuit and reoperated by each of said impulses, equipment conditioned by said initial operation of said tube and thereafter controlled by the reoperations of said tube to direct the extension of a connection in accordance with said impulses.

7. In a telephone system, two exchanges interconnected by a trunk line, means in one of said exchanges operated to transmit direct current impulses over said trunk line to the othe of said exchanges, a gas discharge tube associated with said trunk line in the other of said exchanges and operated intermittently by said impulses, means controlled by said tube for directing the extension of a connection in accordance with said impulses, said connection including a normally disabled speech circuit, means in said one exchange operated after said impulses have been transmitted to transmit a further'impulse over said trunk 1ine, and means operated by said tube responsive to said further impulse to enable said speech circuit.

8. In a telephon system, two exchanges inter connected by a trunk line, means in one of said exchanges operated at will to complete a direct current circuit over said trunk line, an interrupter in said circuit at said one exchange then operated to transmit impulses over said trunk line, a pair of gas discharge tubes in the other of said exchanges, means whereby one of said tubes but not the other operates responsive to the completion of said circuit and said two tubes thereafter operate respectively at the beginning and end of each impulse, equipment conditioned by the initial operation of said one tube and thereafter controlled conjointly by said two tubes to direct the extension of a connection in accordance with said impulses.

9. In a system as claimed in claim 8, wherein each of said tubes has a grid and a cathode, a circuit for heating said cathodes including both said cathodes in series, and means for normally applying to the two grids a slightly different bias to compensate for the difierence in mean potentials of said cathodes, whereby said tubes require signals of substantially equal strength to be applied to their respective grids to bring about the operation of said tubes.

10. In a telephone system, a line, means for impressing direct current impulses upon said line, a pair of grid controlled gas discharge tubes so associated with said line that one of said tubes operates responsive to the beginning of each impulse and the other operates responsive to the end of each impulse, means controlled conjointly by said tubes to direct the extension of a connection in accordance with said impulses, said connection including a disabled speech circuit, means operated after the last of said impulses to impress upon said line such a potential as to reoperate said other tube without first reoperating said one tube, and means thereupon controlled by said other tube to enable said speech circuit.

11. A telephone system as claimed in claim 10, wherein said tubes are inductively connected to said line so that any particular change of current in said line causes potentials of opposite polarity to be impressed simultaneously upon the grids of said two tubes.

2. In a telephone system as claimed in claim 10, wherein said one tube has a normally open anode circuit and said other tube has a normally closed anod circuit, means for controlling said other tube over said line before said impulses are impressed upon said line, thereby to cause said other tube to energize its anode circuit, and means controlled by said energization of the anode circuit of said other tube for causing the anode circuit of said one tube to be closed, thereby to prepare said one tube to respond to the beginning of each impulse.

13. In a telephone system, two exchanges interconnected by a trunk line, said trunk line com- I prising not more than two conductors, an operators position in one of said exchanges, means operated under the control of said position to transmit call directing impulses over said trunk line, impulse receiving equipment in the other of said exchanges normally connected to said trunk line for responding to said impulses and causing a speech path to be extended in accordance therewith, and means operated responsive to an operation performed at said position after the transmission of said call directing impulses for disconnecting said receiving equipment from said trunk line and connecting to said trunk line the extended speech path.

14. In a telephone system, a line, means for impressing direct current impulses upon said line, impulse receiving equipment normally connected to said line, said equipment including a first gas discharge tube operated responsive to the beginning of each impulse and a second gas discharge tube operated responsive to the end of each impulse, meanscontrolled conjointly by said tubes to direct the extension of a speech path, means operated after the last of said impulses to impress upon said line such a potential as to reoperate said second tube without first reoperating said first tube, and means thereupon controlled by said second tube to disconnect said impulse receiving equipment from said line and to connect said speech path to said line.

15. In a telephone system, two exchanges interconnected by a trunk line, means in one of said exchanges operated to transmit direct current impulses of a certain polarity over said trunk line and then operated to transmit a direct current impulse of the opposite polarity over said trunk line, a gas discharge tube in the other of said exchanges, means whereby said tube operates intermittently responsive to the impulses of said certain polarity and causes a connection to be extended in accordance with said impulses, said connection including a disabled speech circuit, and means whereby said tube operates responsive to the impulse of said opposit polarity thereby to enable said speech circuit.

16. In a telephone system, two exchanges interconnected by a trunk line, means in one of said exchanges operated at will to complete a direct current circuit over said trunk line, an interrupter in said circuit at said one exchang then operated to transmit impulses over said trunk line, a pair of gas discharge tubes in the other of said exchanges, means whereby one of said tubes operates responsive to the completion of said circuit and thereafter operates responsive to the end of each impulse, means controlled by the initial operation of said one tube for conditioning the other of said tube for operation whereby said other tube thereafter operates responsive to the beginning of each impulse, and equipment conditioned by the initial operation of said one tube and thereafter controlled conjointly by said two tubes to direct the extension of a connection in accordance with said impulses.

17. In a telephone system, two exchanges interconnected by a trunk line, means in one of said exchanges operated at will to complete a direct current circuit over said trunk line, an interrupter in said circuit at said one exchange then operated to transmit impulses over said trunk line, a transformer in the other of said exchanges having its primary winding connected to said trunk line, a grid-controlled gas discharge tube having its grid connected to a secondary winding of said transformer so that said tube is brought to its critical grid potential responsive to the completion of said circuit and also responsive to the end of each impulse, another grid controlled gas discharge tube having its grid connected to a secondary winding of said transformer so that said other tube is brought to its critical grid potential responsive to the beginning of each impulse, each tube effective to operate when brought to its critical grid potential, equipment conditioned by the operation of said first tube responsive to the initial completion of said circuit and thereafter controlled conjointly by said two tubes to direct the extension of a connection in accordance with said impulses.

18. In combination, a circuit including in series a source of potential, the winding of a relay and a, gas discharge tube, said tube normally preventing said source from causing current to flow over said circuit, means for momentarily applying to said tube such a potential as to initiate the discharge thereof, thereby to cause current to flow over said circuit and operate said relay, a circuit connected by said relay upon operation in bridge of the discharge path through said tube thereby to extinguish said tube, said bridge circuit includ- 12 ing a discharged condenser and, in series therewith, a, source of potential so poled with respect to'said first source as to aid said first source in charging said condenser through the winding of said relay, the condenser charging current hence maintaining said relay operated for an interval after the discharge through said tube has ceased, and said relay eifective to restore to normal when the condenser charging current ceases to flow due 10 to th condenser becoming charged.

REGINALD TAYLOR. GEORGE THOMAS BAICER. 

